TUSSOCK MOTHS: At left is an adult male, at right, a female. Small cocoons and egg mass cases are attached to the underside of twigs and branches.
State and federal forest officials are bracing for a continuation of last year’s Douglas fir tussock moth outbreak that lightly defoliated fir and spruce trees in the Blue Mountains.
A news release from the Oregon State University Extension Office in La Grande said light defoliation was mapped last year across 9,000 acres of the Umatilla National Forest, including 7,800 acres in Washington and 1,200 acres in Oregon.
The release said most of the defoliation occurred in the Wenaha-Tucannon Wilderness area but may spread and increase in severity in 2012.
Mark Jaques, an Oregon Department of Forestry forester working in La Grande, said damage last year was mostly on public lands, with some spilling over from Washington into Union, Wallowa and Umatilla counties in Oregon.
He said the situation bears close watching.
“So far what we’ve seen is light but, in the early 1970s, we had a devastating outbreak. The tussock moth is a critical insect that we watch,” Jaques said.
Jaques said his department has been monitoring several other areas and so far results have been encouraging.
“We’ve put out traps in areas near Cove, Elgin and High Valley near Union, and we haven’t seen a significant increase,” he said.
Tussock moth damage primarily affects grand fir, subalpine fir, Douglas fir and spruce trees. The damage in 2011 was typically light, with the top third of tree crowns primarily affected.
Defoliation was mapped by the U.S. Forest Service, the Oregon Forestry Department and the Washington State Department of Natural Resources.
Prior to 2011, the last outbreak in the Blue Mountains occurred in 2000-2002. Outbreaks typically collapse within two to four years because of a buildup of natural enemies, such as a viral disease or parasites.
Defoliation damage by tussock moths can reduce tree growth, cause top-kill and may predispose some trees to attack by bark beetles. Repeated defoliation is most damaging to trees. Caterpillars feed on both new and old foliage. The agencies warn that hairs found on caterpillars, cocoons and egg masses are a skin irritant for many people.
According to the Oregon Department of Forestry, the Douglas fir tussock moth is a major defoliator of true firs and Douglas fir in the western United States.
Tussock moths occur in most forests in Oregon, but episodes of severe defoliation are restricted to the Blue Mountains of northeast Oregon, and Klamath and Lake counties.
Defoliation by this insect can cause top kill, reduce radial growth and result in up to 40 percent tree mortality.
To reduce potential damage in future outbreaks, susceptible stands can be thinned to favor non-host species and increase spacing between host trees that are retained. Because severely defoliated trees may recover, thinning and salvage is best done following the outbreak.
To evaluate management options for the most recent outbreak, the Washington State Department of Natural Resources and the Oregon Department of Forestry can assist forest and woodlot property owners in the affected areas who observe Douglas fir tussock moth egg masses or tree damage. To report damage, contact Rob Flowers of the Oregon agency, 503-945-7396, or Glen Kohler at the Washington State Department of Resources, 360-902-1342.